This invention relates to power supplies for powering DC positioning motors and, more particularly, in a pen plotter having a DC precision positioner driven by a DC driver normally outputting a voltage having a curve with a dead band and a variable slope which is a function of the voltage input thereto, to the improvement for allowing a non-regulated voltage supply to be used as an input to the DC driver comprising, voltage curve determination means for periodically sensing and determining a present voltage curve for the voltage output from the DC driver; and, compensation voltage application means for inputting a compensation voltage to the DC driver in combination with voltage from the non-regulated voltage supply which will force the voltage output from the DC driver to attain a value on a voltage curve having a constant slope and no deadband.
FIG. 1 preferred circuit 10. The input to the circuit 10 is a non-regulated voltage VNR of 24-36 volts (i.e. 30 volts plus or minus 6 volts) which is transmitted on line 12 via a power MOSFET transistor (power driver) 14 and a power inductor 16 through line 18 to drive the fan motor 20.
The non-regulated voltage VNR is also transmitted via the line 22 and an analog-to-digital (A/D) converter 24 to the digital controller 26 which is provided with look-up tables 28 defining pulse width modulated outputs. Depending on the digitized magnitude of the non-regulated voltage VNR received from the A/D converter 24, the logic contained in the firmware 30 of the controller 26 uses that value as an index to read out a compensation value from the associated entry in the look-up tables 52 and transmits a corresponding pulse width modulated (PWM) output on line 32 to the base of the transistor 34 which, in turn, controls the power MOSFET transistor (power driver) 14. As depicted in FIG. 4, the PWM output on line 32 determines the time T1 out of the total duty cycle, T, that transistors 34 and 14 are on.
When the transistors 34 and 14 are conducting or "on", the current IL flowing through the power inductor 16 charges the capacitor 36. When the transistors 34 and 14 are on for only a part of the duty cycle T, the capacitor 36 discharges. Ideally, the voltage level on line 18 is maintained at 24 volts. When the non-regulated voltage VNR on line 12 is 24 volts, the PWM output from the controller 26 is such that the transistors 34 and 14 are on all of the time. When the non-regulated voltage VNR on line 12 is greater than 24 volts, the PWM output from the controller 26 is such that the transistors 34 and 14 are on for a portion of the duty cycle T as a function of the magnitude of the voltage. In tested embodiments, it was found that by pre-computing the values for the look-up tables 52 and then adjusting them for actual conditions as part of a calibration process, the voltage on line 18 into the fan motor 30 could be maintained effectively at 24 volts within the desired tolerances. The use of look-up tables and the calculations for determining the values of the PWM outputs for particular applications are procedures well known and understood by those skilled in the art which can be accomplished without undue experimentation and, therefore, in the interest of simplicity and the avoidance of redundancy, the details thereof have not been described herein on in that co-pending application.
There are other "motors" within a pen plotter which, if powerable from a non-regulated voltage, could also provide a lower cost-to-build benefit to the manufacturer of the pen plotter. In particular, these include a drum drive motor providing x-axis movement, a carriage drive motor providing y-axis movement, and a pen height motor for lifting, lowering, and positioning a plotting pen in the z-axis. While the above-referenced co-pending application was directed to providing a constant voltage level through the use of firmware control of a non-regulated power source, these other motors are of the DC type wherein the torque of the motor is dependent on the voltage input thereto, wherefore a constant gain or slope of the voltage curve must be maintained in order to perform precision positioning with the motor.
Wherefore, it is the object of this invention to provide an alternate approach to the subject matter of the above-referenced co-pending application which will allow firmware control of a non-regulated power source to provide a voltage output having a constant gain or slope to the voltage curve.